To convert transmitted and received signals from a baseband to a radio frequency or vice versa, radio-frequency mixers are normally used, which require a carrier signal for the frequency conversion. This carrier signal is normally provided by an oscillator, for example a voltage controlled oscillator. However, the oscillator normally does not oscillate at the carrier frequency which is required for the mixer but rather at a different frequency so that reactions by the transmission frequency on the oscillator are reduced. In this context, frequency conversion for the oscillator signal requires attention to be paid to low power consumption and low phase noise in the frequency converter.
The frequency conversion from the oscillator frequency to the carrier frequency that is required for the actual useful-signal frequency converter is effected, by way of example, by the oscillator frequency being mixed, in a frequency mixer, with itself, possibly after phase shifting, or with a signal that has been derived therefrom, divided down and filtered. The oscillator frequency can be divided down using integer division ratios. Filtering the oscillator signal which has already been divided down serves to avoid unwanted signal components at the output of the mixer, that might arise as a result of the mixing of relatively high harmonics. In this case, depending on the design pursued for the transmitter or receiver, the image frequency arising in the mixer for conditioning the oscillator signal may be extinguished as a result of the mixer having been produced using an architecture which suppresses the image frequency. However, this suppression is greatly dependent on the pairing tolerances in the frequency mixer, which in turn make high demands on production tolerances and also result in relatively high power consumption, poor phase noise properties and a large area requirement for the frequency converter, which is complex to produce overall.
The document DE 19710490 A1 specifies a frequency conversion circuit which comprises two frequency mixers. The first frequency mixer multiplies an input signal by a local oscillator signal, and the local oscillator signal is supplied to the second frequency mixer, which is connected downstream of the first, after frequency division.